siamese fighting fish intelligence

Fish suffer pain science proves

There is no published experimental or observational research that proves that fish do not suffer pain.
The argument is put, that because fish do not have the complex outer layer of the brain that humans do (the neocortex), fish cannot feel pain. Birds and amphibians also lack a neocortex, yet many scientific studies have amply demonstrated pain and suffering in these groups.1 It is now known that the telencephalon, in the forebrain, processes pain in fish, along with other brain areas.
Scientists and established bodies now recognise that fish do feel pain.
Dr Lynne Sneddon was the first person to discover pain receptors in fish.
"Fish... demonstrate pain-related changes in physiology and behaviour that are reduced by painkillers; that they exhibit higher brain activity when painfully stimulated." 2
Professor Braithwaite, author of Do Fish Feel Pain?
"The evidence we have to support sentience and pain perception in fish is as good as anything we have for birds and mammals. Fish, like birds and mammals, have a capacity for self-awarenesss." 3
Professor Donald Broom, Emeritus Professor of Animal Welfare, Cambridge University, England.
"Almost all of the characteristics of the mammalian pain system are also described for fish. Emotions, feelings and learning from these are controlled in the fish brain in areas anatomically different but functionally very similar to those in mammals.
The evidence of pain and fear system function in fish is so similar to that in humans and other mammals that it is logical to conclude that fish feel fear and pain. Fish are sentient beings."
The Farm Animal Welfare Committee 2014 report, Opinion on the Welfare of Farmed Fish.
"Fish are able to detect and respond to noxious stimuli, and FAWC supports the increasing scientific consensus that they experience pain." 5
The European Union's, Panel on Animal Health and Welfare European Food Safety Authority:
"Different species of fish have evolved highly sophisticated sensory organs to survive in changing and varied environmental conditions. There is scientific evidence to support the assumption that some fish species have brain structures potentially capable of experiencing pain and fear. The balance of evidence indicates that some fish species have the capacity to experience pain." 6
The Royal Society for the Protection of Animals, in Britain, commissioned in 1980 an independent panel of experts. It said:
"It is only reasonable for mankind to behave on the assumption that all vertebrates are capable of suffering to some degree or another." 7
The American Veterinary Medical Association accepts that fish feel pain:
"Evidence supports the position that finfish should be accorded the same considerations as terrestrial vertebrates in regard to relief from pain." 8

Pain receptors


A fish’s eye is similar to the human eye, with the trigeminal nerve ending in receptors in the outer layer. Twenty-seven pain receptors were found in trout and were as sensitive as those in people. 9
twenty seven pain receptors found in eyes of fish


Experiments in Scotland located fifty-eight receptors in the face and head of rainbow trout. Twenty-two of these were pain receptors, similar to those found in amphibians, birds, and mammals. 10

"This study demonstrates that the rainbow trout is capable of nociception."

twenty two pain receptors in face of fish After Sneddon


In humans and other higher vertebrates, there are two types of nerve fibers used in pain transmission. "A" fibers transmit pain signals quickly; they are involved in the pricking, sensation of pain, while "C" fibers transmit pain signals slowly, and are involved in aching pain.
After fish were deeply anesthetized, the head was operated on to expose the nerve. Then fine wire, heat, and chemicals were used to stimulate the nerve, while recordings were made. Both A and C fibers were found. As in people, the nerve projects into the brain for processing in various regions. 11
sensory nerve
"The present study on the trout has demonstrated that the trigeminal nerve has both C and A-delta fibers."


Russian scientists recorded the responses of various fish to painful electrical shocks. The fish jerked their tails in response. Afterwards, an analgesic was given, which reduced pain sensitivity by up to 89%.
There was sensitivity throughout the whole body, comparable to humans. The most sensitive areas were the tail and pectoral fins, skin around the eye, and olfactory sacs. 12


In Ireland, researchers subjected goldfish to heat, pinpricks, and pressure.
"This study has shown that there is neuronal activity in all brain areas including the telencephalon, suggesting a nociceptive pathway from the periphery to the higher central nervous system of fish." 13
In further research, carried out in England, the faces of trout were stimulated while responses in the trigeminal nerve in the brain were recorded. It was discovered that skin receptors of trout are more sensitive to mechanical stimulus, and heat, than mammals and birds.
"Trout cutaneous receptors recorded here are more sensitive overall to mechanical stimuli than those of mammals and birds, with some responding to stimuli as low 0.001g." 14

Brain structures

In ourselves, sensory nerve fibers send electrical signals from the site of the stimulus to the spinal cord and then to the brain. Signals reach the thalamus, via the brain stem. The thalamus is a sort of relay station, which forwards signals for processing in different areas of the brain. One of these areas is the limbic system, which is the emotional center of the brain. The neocortex, on the outer surface of the brain, handles conscious thought in the perception of pain.
All of these areas are present in fish, or the fish have the same function elsewhere in their brain.
Fish pain specialist, Dr. Braithwaite, told the Los Angeles Times that, although simpler than ours, the brain of a fish is more similar than once thought.
"Their brains are not as different from ours as we once thought. Although less anatomically complex than our own brain, the function of two of their forebrain areas is very similar to the mammalian amygdala and hippocampus - areas associated with emotion, learning, and memory. If these regions are damaged in fish, their learning and emotional capacities are impaired; they can no longer find their way through mazes, and they lose their sense of fear." 15


Norwegian scientists applied electrical shocks to anesthetized salmon. They traced the nerve pathway to the telencephalon in the brain.

"Our results add to an increasing body of studies indicating that fish have many of the prerequisites for complex processing of external events." 16

The telencephalon in fish is probably equivalent to the hippocampus and amygdala in humans, and the outer layer of the telencephalon, the pallium, equivalent to our neocortex.

At the University of Sevilla, damage was caused to the telencephalon of goldfish. This resulted in the fish being unable to learn to avoid an electric shock. Similar damage to the amygdala and hippocampus of mammals produces the same effect. It was concluded that fish and mammals have inherited the same brain functions from an ancient ancestor. 17,18

fish and human-brain compared


The German magazine, Der Spiegel in 2011, reported research that showed that the human limbic system has its equivalent in the fish’s telencephalon. However, this area was not found in fish before, as rather than being on the inside, as in the human brain, it moves to the outside after embryonic development.
der spiegel where%20senses are interpreted


It can be demonstrated that fish feel pain by blocking their reaction to it with analgesic drugs (painkillers).


In New Orleans, biologists applied electricity to fish which produced an "agitated swimming response". After they were injected with morphine, a strong analgesic in mammals, the voltage had to be increased to achieve the same response. However, when naloxone, a medicine that blocks opioids, was injected, the analgesic effect of morphine was blocked. 19

In Norway, scientists gradually applied greater heat to the skin of goldfish. The fish showed an escape response at a particular temperature threshold. Later, when morphine, which dampens pain in mammals, was injected, the temperature had to rise significantly higher to evoke the same response. 20


In an experimental tank, zebrafish chose to live where there were gravel and plants, instead of a barren environment. However, when they were injected with a painful chemical, they preferred to spend time in the barren environment, where there was analgesia in the water.

"Those fish experiencing pain lost their preference for the enriched chamber and were willing to spend more time in the barren, unfavorable area only if analgesia was present."

"This demonstrates that the fish were willing to pay the cost of being in an unpreferred environment to obtain analgesia, and thus it can be inferred that these fish must have obtained some reward possibly in terms of pain relief such that the pain was reduced." 21


Unfortunate zebrafish are now commonly used to study pain.
In Portugal, zebrafish were used in an experiment where morphine was given to relieve the pain from acetic acid.

"Fish possess the necessary sensory components to detect a noxious stimulus and also have brain areas similar to those of higher vertebrates that respond to painful stimuli."

"There is convincing evidence that fish possess nociceptors that react in a fashion similar to those in mammals." 22

At only five days after being fertilized, zebrafish larvae were exposed to weak acid, causing them to slow their swimming. However, when painkillers - aspirin, lidocaine, or morphine - were dissolved in their tank, before the acid was later added, they swam normally. 23


Opioid receptors were present in fish over 450 million years ago, allowing fish to survive pain. 24
Researchers at Liverpool University, in England, reported that opiate receptors were found in fish as well as an enkephalin-like substance, which is involved in regulating pain, in various brain areas of goldfish, catfish, African lungfish, and rainbow trout.

"Opiate receptors have been found in fish as well as enkephalin like substances in various brain areas of goldfish, catfish, African lungfish and rainbow trout." 25

Remembering pain


Sticklebacks receive some protection from predator fish through their spines. In 1957 at Oxford University, England, researchers found pike and perch initially snapped them up but then rejected them. After a few experiences, the pike and perch learned to avoid the sticklebacks altogether. When the stickleback spines were removed, their protection disappeared.
After very few experiences both perch and pike become negatively conditioned to the sight of sticklebacks and avoid them before they have made contact.

"Sticklebacks are rejected when, after being snapped up, their spines hurt the predator’s mouth" 26

Dutch researchers used anglers to fish for pike for three days, using live bait or spinner hooks. The fish had never been fished before.

Pike hooked once by a spinner rarely took it again, and would avoid spinners for the rest of the five days.

Individual fish, and then the whole population of the two pools, learned, and then remembered, that the spinner was to be avoided. This memory lasted for at least one year.

spinner used for catching fish
The same researchers found that when carp were hooked and then lost, not only did these fish later become nearly uncatchable but also other carp, who were not caught, also became so. Even after one year, they were three times more difficult to catch.
"It was difficult to capture pike more than once by spinning."


At Missouri University, goldfish were placed in a shuttlebox. They were trained to respond if light colors at each end were the same by swimming to the other end. When they were wrong, they received an electric shock. Later, they had to respond if the colors of the lights were different. They learned and remembered how to avoid the shocks.

"These findings are consistent with the interpretation that goldfish learned the original conditional discrimination at a conceptual level, and learned about the general matching or oddity relationships between colors, rather than about a specific set of colors." 27



Goldfish and trout were given mild electric shocks when they entered a particular part of their tank. They swam away, their respiration went up, and cortisol, an indicator of stress, went up in their blood. They learned to spend time at the safe end of the tank, away from the electric shocks, especially on the second day.

"This plasticity in learning suggests that pain responses are not just confined to lower brain reflex actions, but to some degree, involve perception."

"Current legislation is written under the assumption that fish are incapable of perceiving pain and if there is any possibility that this is incorrect, new legislation will have to be developed to improve aquaculture conditions and change fishing practices." 28

Showing pain


Although fish are normally silent, Russian scientists found that wounding European weather loach caused them to vocalize, using their swim bladders.

"According to these parameters, this pain 'shout' differed from sounds that accompanied food seizure, feeding, or spawning behavior." 29


Fish will often not swim close to an object not encountered before. Researchers in Scotland found that trout, that had acetic acid injected into their lips, were less wary than fish that had not had the injection when a lego tower was placed in their tank. They showed their normal cautious response if given morphine later.
The investigators said that the painful acetic acid distracted the attention of the fish. For this to happen, the fish must be consciously aware of the negative experience of the pain.

"It has also been demonstrated that the trout has a prolonged negative behavioural and physiologic response to a noxious event, and this was also seen in the present study. Other fish species can learn to avoid noxious stimuli such as electric shock. The administration of an analgesic reduces these responses to almost normal. Together with the results of this study, the criteria for animal pain have been fulfilled for the trout." 30


In England, acetic acid was injected into the lips of trout.
After their experience, when individual fish were returned to a familiar social group they showed reduced aggression. When the fish were returned to an unfamiliar social group, they showed the usual level of aggression, suggesting that maintaining dominance status took priority over showing signs of pain.
"Our findings provide new evidence that fish are considerably affected by pain ... and that the perception is not just a simple nociceptive reflex."
"These findings may reflect an ability to prioritize motivational drivers in fish, and as such provide evidence for central processing of pain rather than merely showing a nociceptive reflex."


In Scotland, either bee venom or acetic acid was injected into the lips of rainbow trout. A control group was handled but received no injection.
There was a near doubling of respiration, cessation of feeding for 3 hours, and reduced swimming. Back and forth, they balanced on either pectoral fin, while resting on the gravel, and rubbed their lips into the gravel and tank walls.

"These pain-related behaviours are not simple reflexes and therefore there is the potential for pain perception in fish." 32


Karl von Frisch in the 1930s discovered that wounded minnows released a chemical that caused fear in other minnows. Present-day biologists in Singapore isolated the chemical in zebrafish.

"When one fish is injured, others nearby may dart, freeze, huddle, swim to the bottom or leap from the water."

"At low concentrations, the fish were "mildly perturbed," ... "At high concentrations, they stopped darting altogether and froze in place for an hour or longer." 33

(1) Animal Welfare Perspectives on Recreational Angling, Cooke, S.J. and Sneddon, L.U., 2007. Animal welfare perspectives on recreational angling. Applied Animal Behaviour Science, 104(3-4), pp.176-198
(2) Sneddon, L.U., 2015. Pain in aquatic animals. Journal of Experimental Biology, 218(7), pp.967-976.
(3) Do Fish Feel Pain?
(4) Broom, D.M., 2016. Fish brains and behaviour indicate capacity for feeling pain. Animal Sentience: An Interdisciplinary Journal on Animal Feeling, 1(3), p.4.
(5) Farm Animal Welfare Committee, Opinion on the Welfare, of Farmed Fish, February 2014 (pdf)
(6) European Food Safety Authority (EFSA), 2009. General approach to fish welfare and to the concept of sentience in fish. EFSA Journal, 7(2), p.954
(7) Report of the Panel of Enquiry into Shooting and Angling (The Medway Report), Chairman: Lord Medway, 1980
(8) AVMA Guidelines for the Euthanasia of Animals American Veterinary Medical Association (pdf)
(9) Properties of corneal receptors in a teleost fish, Properties of corneal receptors in a teleost fish, Sneddon, Ashley, Mccrohan, 2006 (pdf)
(10) Nociception in fish: stimulus–response properties of receptors on the head of trout Oncorhynchus mykiss, Sneddon, Ashleya, McCrohan, 2007
(11) Anatomical and electrophysiological analysis of the trigeminal nerve in a teleost fish, Oncorhynchus mykiss, Sneddon, 2002
(12) Pain sensitivity of fishes and analgesia induced by opioid and nonopioid agents, Chervova, Lapshin, 2004 (pdf)
(13) Mechanoreceptive and nociceptive responses in the central nervous system of goldfish (Carassius auratus) and trout (Oncorhynchus mykiss), Dunlop, Laming, 2005
(14) Nociception in fish: stimulus–response properties of receptors on the head of trout Oncorhynchus mykiss, Sneddon, 2007
(15) Los Angeles Times, 8 October 2006
(16) Somatosensory evoked potentials in the telencephalon of Atlantic salmon (Salmo salar) following galvanic stimulation of the tail, Nordgreen, Horsberg, Ranheim, Chen, 2007 (pdf)
(17) Avoidance Response in Goldfish: Emotional and Temporal Involvement of Medial and Lateral Telencephalic Pallium | Portavella, Torres, Salas, 2004 (pdf)
(18) Neural Mechanisms of Learning in Teleost Fish, Rodriguez, Broglio, Duran, Gomez, Salas, 2006
(19) Similar antagonism of morphine analgesia by MIF-1 and naloxone in Carassius auratus, Ehrensing, Michell, Kastin, 1981
(20) Thermonociception in fish: Effects of two different doses of morphine on thermal threshold and post-test behaviour in goldfish (Carassius auratus), Nordgreen, Garnerb, Janczak, Ranheima, Muirb, EinarHorsberga, 2009
(21) Pain Perception in Fish, Sneddon, 2011
(22) A Novel Behavioral Fish Model of Nociception for Testing Analgesics , Correia, Cunha, Scholze, Stevens, 2011 (pdf)
(23) Reduction in activity by noxious chemical stimulation is ameliorated by immersion in analgesic drugs in zebrafish, Sneddon, Lopez-Luna, Al-Jubouri, Al-Nuaimy
(24) Stefano, G.B. and Kream, R.M., 2010. Opioid peptides and opiate alkaloids in immunoregulatory processes. Archives of medical science: AMS, 6(3), p.456
(25) Evolution of nociception in vertebrates: comparative analysis of lower vertebrates, Sneddon, 2004
(26) The Spines of Sticklebacks (Gasterosteus and Pygosteus) as Means of Defence against Predators (Perca and Esox), Hoogland, Morris, Tinbergen, 1957
(27) Matching and oddity conditional discrimination in the goldfish as avoidance responses: Evidence for conceptual avoidance learning, Zerbolio, 1983
(28) Mechanoreceptive and nociceptive responses in the central nervous system of goldfish (Carassius auratus) and trout (Oncorhynchus mykiss), Dunlop, R, Laming, P, 2006
(29) Pain sensitivity of fishes and analgesia induced by opioid and nonopioid agents, Chervova1, Lapshin (pdf)
(30) Novel object test: examining nociception and fear in the rainbow trout, Sneddon, Braithwaite, Gentle, 2003
(31) Effect of noxious stimulation upon antipredator responses and dominance status in rainbow trout, Sneddon, Ashley, Ringrosea, Edwards, Wallington, McCrohan, 2003
(32) Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system, Sneddon, Braithwaite, Gentle, 2003 (pdf)
(33) Chondroitin Fragments Are Odorants that Trigger Fear Behavior in Fish, Mathuru, Kibat, Cheong, Shui, Wenk, Friedrich, Jesuthasan, 2012
Fish Pain